Semiconductor device and manufacturing method thereof

ABSTRACT

Disclosed is a semiconductor device including an oxide semiconductor film. A first oxide semiconductor film with a thickness of greater than or equal to 2 nm and less than or equal to 15 nm is formed over a gate insulating layer. First heat treatment is performed so that crystal growth from a surface of the first oxide semiconductor film to the inside thereof is caused, whereby a first crystal layer is formed. A second oxide semiconductor film with a thickness greater than that of the first oxide semiconductor film is formed over the first crystal layer. Second heat treatment is performed so that crystal growth from the first crystal layer to a surface of the second oxide semiconductor film is caused, whereby a second crystal layer is formed. Further, oxygen doping treatment is performed on the second crystal layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/110,382, filed May 18, 2011, now allowed, which claims the benefit ofa foreign priority application filed in Japan as Serial No. 2010-117020on May 21, 2010, both of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device including anoxide semiconductor and a manufacturing method thereof.

In this specification, a semiconductor device generally means a devicewhich can function by utilizing semiconductor characteristics, and anelectrooptic device, a semiconductor circuit, and an electronicappliance are all semiconductor devices.

2. Description of the Related Art

In recent years, attention has been focused on a technique for formingthin film transistors (TFTs) using a thin semiconductor film (with athickness of from several tens of nanometers to several hundreds ofnanometers, approximately) formed over a substrate having an insulatingsurface. Thin film transistors are widely applied to electronic devicessuch as an IC and an electro-optical device and have been expected to berapidly developed particularly as switching elements for an imagedisplay device. Various metal oxides are used for a variety ofapplications.

Some metal oxides have semiconductor characteristics. The examples ofsuch metal oxides having semiconductor characteristics are a tungstenoxide, a tin oxide, an indium oxide, a zinc oxide, and the like. A thinfilm transistor whose channel formation region includes such metaloxides having semiconductor characteristics is already known (PatentDocuments 1 and 2).

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055

SUMMARY OF THE INVENTION

However, the electric conductivity of an oxide semiconductor changeswhen hydrogen or water forming an electron donor enters the oxidesemiconductor during a process for manufacturing a device. Such aphenomenon becomes a factor of variation in the electric characteristicsof a transistor including the oxide semiconductor.

In view of the above problems, one object is to provide a semiconductordevice including an oxide semiconductor film, which has stable electriccharacteristics and high reliability.

One embodiment of the disclosed invention is a method for manufacturinga semiconductor device including the following steps: forming a gateelectrode layer; forming a gate insulating layer over the gate electrodelayer; forming a first oxide semiconductor film with a thickness ofgreater than or equal to 2 nm and less than or equal to 15 nm over thegate insulating layer; forming a first crystal layer by performing firstheat treatment so that crystal growth from a surface of the first oxidesemiconductor film to an inside of the first oxide semiconductor film iscaused; forming a second oxide semiconductor film with a thicknessgreater than that of the first oxide semiconductor film over the firstcrystal layer; forming a second crystal layer by performing second heattreatment so that crystal growth from the first crystal layer to asurface of the second oxide semiconductor film which is over the firstcrystal layer is caused; forming a source electrode layer and a drainelectrode layer over a stacked layer of the first crystal layer and thesecond crystal layer; and performing oxygen doping treatment to supplyan oxygen atom to the second crystal layer after forming the secondcrystal layer.

Note that in the above manufacturing method, a high-density oxidesemiconductor film can be obtained by crystallization through the firstheat treatment or the second heat treatment. Then, oxygen dopingtreatment is performed on the high-density oxide semiconductor film tosupply an oxygen atom to the oxide semiconductor film, and performingheat treatment on the oxide semiconductor film to which the oxygen atomis supplied.

Another embodiment of the disclosed invention is a method formanufacturing a semiconductor device including the following steps:forming a gate electrode layer; forming a gate insulating layer over thegate electrode layer; performing halogen doping treatment on the gateinsulating layer; forming a first oxide semiconductor film with athickness of greater than or equal to 2 nm and less than or equal to 15nm over the gate insulating layer: forming a first crystal layer byperforming first heat treatment so that crystal growth from a surface ofthe first oxide semiconductor film to the inside of the first oxidesemiconductor film is caused; forming a second oxide semiconductor filmwith a thickness greater than that of the first oxide semiconductor filmover the first crystal layer: forming a second crystal layer byperforming second heat treatment so that crystal growth from the firstcrystal layer to a surface of the second oxide semiconductor film whichis over the first crystal layer is caused; and forming a sourceelectrode layer and a drain electrode layer over a stacked layer of thefirst crystal layer and the second crystal layer.

Note that in the above manufacturing method, oxygen might be releasedfrom the gate insulating layer depending on the conditions of the firstheat treatment; therefore, it is useful to prevent reduction of oxygenin the gate insulating layer by adding chlorine with a large mass to thegate insulating layer before the first heat treatment. Further, oxygendoping treatment may be performed after the second heat treatment.

In this specification, a halogen element means an element belonging toGroup 17 in the periodic table (such as fluorine (F), chlorine (CO,bromine (Br), or iodine (I)); typically, fluorine or chlorine can beused, and one kind of halogen element or a plurality of kinds of halogenelements may be used.

A halogen element is added to a gate insulating layer by performinghalogen doping treatment in a gas atmosphere containing a halogenelement. Note that the above “halogen doping” means to add halogentypified by chlorine and fluorine to a bulk. For example, in the casewhere chlorine is used as halogen, at least one of a chlorine radical, achlorine atom, and a chlorine ion is added to a bulk. The term “bulk” isused in order to clarify that halogen is added to the inside of a thinfilm in addition to a surface of the thin film. Further, the term“halogen doping” includes “halogen plasma doping” by which halogen whichis made to be plasma is added to a bulk. In addition, a halogen elementcan be contained in the gate insulating layer when the gate insulatinglayer is deposited.

One of the characteristics of each of the above manufacturing methods isthat the first crystal layer is c-axis aligned perpendicularly to thesurface thereof. One of the characteristics of each of the abovemanufacturing methods is that the second crystal layer is c-axis alignedperpendicularly to the surface thereof.

Note that the above “oxygen doping” means that oxygen (which includes atleast one of an oxygen radical, an oxygen atom, and an oxygen ion) isadded to a bulk. The term “bulk” is used in order to clarify that oxygenis added to the inside of a thin film in addition to a surface of thethin film. Further, the term “oxygen doping” includes “oxygen plasmadoping” by which oxygen which is made to be plasma is added to a bulk.

By the above oxygen doping treatment, oxygen whose amount is greaterthan the stoichiometric proportion exists in at least one of the oxidesemiconductor film (a bulk thereof), the insulating film (a bulkthereof), and an interface between the oxide semiconductor film and theinsulating film. The amount of oxygen is preferably greater than thestoichiometric proportion and less than four times of the stoichiometricproportion, more preferably greater than the stoichiometric proportionand less than double of the stoichiometric proportion. Here, as anexample of an oxide containing excessive oxygen whose amount is greaterthan the stoichiometric proportion, an oxide which satisfiesg>3a+3b+2c+4d+3e+2f can be given when the composition of the oxide isexpressed by In_(a)Ga_(b)Zn_(c)Si_(d)Al_(e)Mg_(f)O_(g) (a, b, c, d, e,f, g≧0) in appearance. An oxide containing composition other than theabove may be considered in a similar manner. Note that oxygen added bythe oxygen doping treatment may exist between lattices of the oxidesemiconductor.

One embodiment of the present invention is a semiconductor deviceobtained by the above method for manufacturing a semiconductor device,in which the halogen doping treatment is performed. The semiconductordevice includes the following components: a gate insulating layer whichcontains a halogen element and which is provided over a gate electrodelayer; a first crystal layer which has an a-b plane parallel to asurface and which is c-axis aligned perpendicularly to the surface andprovided over the gate insulating layer; a second crystal layer which isc-axis aligned perpendicularly to a surface and provided over and whichis in contact with the first crystal layer; and a source electrode layerand a drain electrode layer provided over a stacked layer of the firstcrystal layer and the second crystal layer. In addition, each of thefirst crystal layer and the second crystal layer is an oxidesemiconductor film.

As a material of the oxide semiconductor films that are the firstcrystal layer and the second crystal layer, any of the followingmaterials can be used: a four-component metal oxide such as anIn—Sn—Ga—Zn—O-based material; three-component metal oxides such as anIn—Ga—Zn—O-based material, an In—Sn—Zn—O-based material, anIn—Al—Zn—O-based material, a Sn—Ga—Zn—O-based material, anAl—Ga—Zn—O-based material, and a Sn—Al—Zn—O-based material;two-component metal oxides such as an In—Zn—O-based material, aSn—Zn—O-based material, an Al—Zn—O-based material, a Zn—Mg—O-basedmaterial, a Sn—Mg—O-based material, an In—Mg—O-based material, and anIn—Ga—O-based material; a single-component metal oxide such as anIn—O-based material and a Sn—O-based material; and the like. Inaddition, the above materials may contain SiO₂. Here, for example, anIn—Ga—Zn—O-based material means an oxide containing indium (In), gallium(Ga), and zinc (Zn), and there is no particular limitation on thecomposition ratio thereof. Further, the In—Ga—Zn—O-based oxidesemiconductor may contain an element other than In, Ga, and Zn.

In the case where an In—Zn—O-based material is used as an oxidesemiconductor, a target therefor has a composition ratio of In:Zn=50:1to 1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio),preferably In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2in a molar ratio), further preferably In:Zn=15:1 to 1.5:1 in an atomicratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). For example, in a targetused for formation of an In—Zn—O-based oxide semiconductor which has anatomic ratio of In:Zn:O=X:Y:Z, the relation of Z>1.5X+Y is satisfied.

An entrapment vacuum pump is preferably used for evacuating a depositionchamber when the gate insulating layer and/or the first crystal layerand/or the second crystal layer, and/or the insulating layer are/ismanufactured in the above method for manufacturing a semiconductordevice. For example, a cryopump, an ion pump, or a titanium sublimationpump is preferably used. The above entrapment vacuum pump functions soas to reduce the amount of hydrogen, water, hydroxyl, or hydridecontained in the gate insulating layer and/or the oxide semiconductorfilm and/or the insulating layer.

The amount of change in the threshold voltage of a transistor includingan oxide semiconductor film which is crystallized and made highly densecan be reduced between before and after a bias-temperature stress (BT)test, whereby a semiconductor device using an oxide semiconductor whichhas stable electric characteristics can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1F are cross-sectional views showing steps of one embodimentof the present invention.

FIGS. 2A to 2F are cross-sectional views showing steps of one embodimentof the present invention.

FIGS. 3A and 3B are cross-sectional views each showing one embodiment ofthe present invention.

FIGS. 4A to 4C are diagrams each showing one embodiment of asemiconductor device.

FIG. 5 is a diagram showing one embodiment of a semiconductor device.

FIG. 6 is a diagram showing one embodiment of a semiconductor device.

FIGS. 7A and 7B are diagrams showing an electronic appliance.

FIGS. 8A to 8F are diagrams each showing an electronic appliance.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways. The present invention is notconstrued as being limited to the description of the embodimentsdescribed below.

(Embodiment 1)

In this embodiment, a method for manufacturing a semiconductor devicewill be described with reference to FIGS. 1A to 1F.

First, a conductive film is formed over a substrate 100 having aninsulating surface, and then a gate electrode layer 112 is formedthrough a photolithography step (see FIG. 1A). Note that a resist maskmay be formed by an inkjet method. Formation of the resist mask by aninkjet method needs no photomask; thus, manufacturing cost can bereduced.

There is no particular limitation on the property of a material and thelike of the substrate 100 as long as the material has at least heatresistance high enough to withstand heat treatment performed later. Forexample, a glass substrate, a ceramic substrate, a quartz substrate, ora sapphire substrate can be used as the substrate 100. Alternatively, asingle crystal semiconductor substrate or a polycrystallinesemiconductor substrate made of silicon, silicon carbide, or the like; acompound semiconductor substrate made of silicon germanium or the like;an SOI substrate; or the like may be used as the substrate 100. Stillalternatively, any of these substrates provided with a semiconductorelement may be used as the substrate 100.

An insulating film serving as a base film may be provided between thesubstrate 100 and the gate electrode layer 112. The base film has afunction of preventing diffusion of an impurity element from thesubstrate 100, and can be formed to have a single-layer structure or astacked-layer structure using one or more films selected from a siliconnitride film, a silicon oxide film, a silicon nitride oxide film, and asilicon oxynitride film.

The gate electrode 112 can be formed by a plasma CVD method, asputtering method, or the like. The gate electrode layer 112 can beformed to have a single-layer structure or a stack-layer structure usinga metal material such as molybdenum, titanium, tantalum, tungsten,aluminum, copper, neodymium, or scandium, or an alloy material whichcontains any of these materials as a main component.

Next, a gate insulating layer 102 is formed over the gate electrode 112layer (see FIG. 1A).

An insulating material whose etching rate in etching for a selectiveetching of an oxide semiconductor film is significantly different fromthat of an oxide semiconductor film formed later is preferably used forthe gate insulating layer 102. The gate insulating layer 102 can beformed with a single-layer structure or a stacked-layer structure usingsilicon oxide, silicon nitride, silicon oxynitride, silicon nitrideoxide, aluminum oxide, aluminum nitride, aluminum oxynitride, aluminumnitride oxide, hafnium oxide, or a mixed material thereof by a plasmaCVD method, a sputtering method, or the like. Considering that the gateinsulating layer 102 functions as a gate insulating layer of atransistor, a material having a high dielectric constant such as hafniumoxide, tantalum oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y)(x>0, y>0)), hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)), hafniumsilicate to which nitrogen is added, or hafnium aluminate to whichnitrogen is added may be employed. A sputtering method is preferable interms of low possibility of entry of hydrogen, water, and the like.

Alternatively, a stacked-layer of an insulating film formed of the abovematerial and a gallium oxide film may be used as the gate insulatinglayer 102.

Next, treatment with halogen 180 (also referred to as halogen dopingtreatment or halogen plasma doping treatment) is performed on the gateinsulating layer 102, whereby halogen is contained in the gateinsulating layer 102 (see FIG. 1B). Chlorine, fluorine, or the like canbe used as the halogen 180. Since the electronegativity of halogen suchas chlorine or fluorine is large, a hydrogen ion which causesdeterioration in a transistor can be trapped. When halogen such aschlorine is contained in the gate insulating layer 102, hydrogen in thegate insulating layer 102 is fixed, and diffusion of the hydrogen fromthe gate insulating layer 102 into the oxide semiconductor film, whichis formed over and in contact with the gate insulating layer 102 in alater step, can be prevented. Consequently, deterioration in thecharacteristics of a transistor is suppressed or reduced even when lightirradiation is performed on the transistor and BT stress is appliedthereto.

Here, since the atomic radius of chlorine is larger than that offluorine and the diffusion coefficient of chlorine is smaller than thatof fluorine, a hydrogen ion is easily fixed in the gate insulating layer102 by chlorine. In particular, when heat treatment is performed later,chlorine is less likely to move compared to fluorine; therefore, ahydrogen ion can be trapped more effectively. Consequently, chlorine ispreferably used as the halogen 180. In this embodiment, chlorine is usedas the halogen 180. In the case where chlorine is used as the halogen180, at least any of a chlorine radical, a chlorine atom, and a chlorineion is included in the halogen 180.

The above-described treatment with the halogen 180 can be performed by aplasma generating apparatus or an ozone generating apparatus.Specifically, for example, the gate insulating layer 102 can beprocessed by generating the halogen 180 using an apparatus capable ofperforming etching treatment on a semiconductor device, an apparatuscapable of performing ashing on a resist mask, or the like. Note thataddition of the halogen is preferably performed under a condition thatdamage to a surface of the gate insulating layer 102 is minimized.

In order to add halogen more preferably, it is preferable to apply anelectrical bias to a substrate when treatment with halogen is performed.Halogen can be deeply added by increasing the bias applied to thesubstrate.

In the case where chlorine is added using an inductively coupled plasma(ICP) apparatus, it is preferable to apply high frequency power supplyof greater than or equal to 1 kW and less than or equal to 10 kW to anICP coil that is a plasma generation source, and to keep a state wherethe plasma is generated for a certain period (greater than or equal to30 seconds and less than or equal to 600 seconds). For example, chlorinedoping treatment may be performed under conditions that ICP power is6000 W, bias power is 250 W, chlorine gas flow rate is 500 sccm,pressure of a treatment chamber is 1.3 Pa, and treatment time is 60seconds.

Halogen may be added by performing irradiation with a halogen ionaccelerated by an electric field. In addition, a high-density plasma CVDmethod using microwaves (for example, a frequency of 2.45 GHz) may beemployed to add halogen. In the case where a high-density plasma CVDmethod using microwaves is employed, a stacked layer of oxidesemiconductor films is hardly damaged at the addition of halogen.

Oxygen may be added at the same time as halogen typified by chlorine.

Next, a first oxide semiconductor film 108 a with a thickness of greaterthan or equal to 2 nm and less than or equal to 15 nm is formed over thegate insulating layer 102 so as to overlap with the gate electrode layer112 (see FIG. 1C). Note that the first oxide semiconductor film 108 a isa film which is subjected to first heat treatment for crystallizationafter the deposition of an oxide semiconductor film and is crystallized.

In this embodiment, the first oxide semiconductor layer with a thicknessof 5 nm is deposited in an oxygen atmosphere, an argon atmosphere, or amixed atmosphere of argon and oxygen. The first oxide semiconductorlayer is deposited with the use of an oxide semiconductor target (anIn—Ga—Zn—O-based oxide semiconductor target (In₂O₃:Ga₂O₃:ZnO=1:1:2[molar ratio])) and the distance between the substrate and the target is170 mm, the pressure is 0.4 Pa, and the direct current (DC) power sourceis 0.5 kW. In this embodiment, since crystallization is intentionallycaused by performing heat treatment in a later step, it is preferable touse an oxide semiconductor target in which crystallization is easilycaused. Further, the first oxide semiconductor layer is deposited whilethe substrate to be processed is held in a deposition chamber kept underreduced pressure, and the substrate temperature is set to higher than orequal to 100° C. and lower than or equal to 600° C., preferably higherthan or equal to 200° C. and lower than or equal to 500° C., morepreferably higher than or equal to 300° C. and lower than or equal to500° C. When the first heat treatment is performed for crystallization,the temperature is set to higher than or equal to 450° C. and lower thanor equal to 850° C., preferably higher than or equal to 600° C. andlower than or equal to 700° C. Heating time is greater than or equal to1 minute and less than or equal to 24 hours. Thus, the first oxidesemiconductor film 108 a having a crystal layer obtained by crystalgrowth from a surface by the first heat treatment is formed.

The crystal layer of the first oxide semiconductor film 108 a is aplate-like crystal obtained by crystal growth from a surface to aninside. The crystal layer has an average thickness of greater than orequal to 2 nm and less than or equal to 10 nm. The crystal layer formedat the surface is c-axis aligned perpendicularly to the surface.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in nitrogen, oxygen, or a raregas such as helium, neon, or argon. It is preferable that the purity ofnitrogen, oxygen, or a rare gas such as helium, neon, or argonintroduced to the heat treatment apparatus be greater than or equal to6N (99.9999%), more preferably greater than or equal to 7N (99.99999%)(that is, the impurity concentration is less than or equal to 1 ppm,more preferably less than or equal to 0.1 ppm). Further, the first heattreatment may be performed in a dry air atmosphere with an H₂Oconcentration of less than or equal to 20 ppm.

Next, a second oxide semiconductor film 108 b whose thickness is atleast greater than that of the first oxide semiconductor film 108 a andless than or equal to 10 μm is formed over the first oxide semiconductorfilm 108 a (see FIG. 1D). A second oxide semiconductor layer isdeposited while the substrate to be processed is held in a depositionchamber kept under reduced pressure, and the substrate temperature isset to higher than or equal to 100° C. and lower than or equal to 600°C., preferably higher than or equal to 200° C. and lower than or equalto 500° C., more preferably higher than or equal to 300° C. and lowerthan or equal to 500° C. Note that the second oxide semiconductor film108 b is a film which is subjected to second heat treatment forcrystallization after the deposition of an oxide semiconductor film andis crystallized. In the second heat treatment, crystal growth is causedwith the use of the crystal layer of the first oxide semiconductor film108 a as a seed. The temperature of the second heat treatment is set tohigher than or equal to 450° C. and lower than or equal to 850° C.,preferably higher than or equal to 550° C. and lower than or equal to650° C. Heating time is greater than or equal to 1 minute and less thanor equal to 24 hours.

It is preferable that the first oxide semiconductor film 108 a and thesecond oxide semiconductor film 108 b be formed using materialscontaining the same components or have the same crystal structure andclose lattice constants (a difference of lattice constants is less thanor equal to 1%). In the case where the materials containing the samecomponents are used, crystal growth can be easily caused with the use ofthe crystal layer of the first oxide semiconductor film 108 a as a seedin crystallization performed later. In addition, in the case where thematerials containing the same components are used, an interface propertysuch as adhesion or electric characteristics is good.

In the stacked layer of the first oxide semiconductor film 108 a and thesecond oxide semiconductor film 108 b, a region overlapping withdepressions and projections of the gate insulating layer has a grainboundary and therefore becomes a polycrystalline body. A region of thestacked layer of the oxide semiconductor films serving as a channelformation region has at least a flat surface and has a crystal structurein which the c-axis of the first oxide semiconductor film 108 a and thec-axis of the second oxide semiconductor film 108 b are aligned. Inaddition, in the stacked layer of the oxide semiconductor films, thea-axis and b-axis of crystals in the polycrystalline body in the channelformation region each are misaligned in some cases.

Next, the stacked layer of the first oxide semiconductor film 108 a andthe second oxide semiconductor film 108 b is processed and anisland-shaped oxide semiconductor film is formed (see FIG. 1E).

The stacked layer of the oxide semiconductor films can be processed bybeing etched after a mask having a desired shape is formed over thesecond oxide semiconductor film 108 b. The mask can be formed by amethod such as photolithography.

Next, treatment with oxygen (also referred to as oxygen doping treatmentor oxygen plasma doping treatment) may be performed on the stacked layerof the oxide semiconductor films if needed. Note that it is preferableto apply an electrical bias to the substrate in order to perform oxygendoping more favorably. Further, heat treatment may be performed on thestacked layer of the oxide semiconductor films subjected to the oxygendoping treatment. The heat treatment temperature is higher than or equalto 250° C. and lower than or equal to 700° C., preferably higher than orequal to 400° C. and lower than or equal to 600° C., or lower than thestrain point of the substrate. The oxygen doping treatment and the heattreatment may be repeated. By repeatedly performing the oxygen dopingtreatment and the heat treatment, the transistor can have higherreliability. Note that the number of repetitions can be setappropriately.

Next, a conductive film for forming a source electrode layer and a drainelectrode layer (including a wiring formed in the same layer as thesource electrode and the drain electrode) is formed over the gateinsulating layer 102 and the stacked layer of the oxide semiconductorfilms and processed to form a source electrode layer 104 a and a drainelectrode layer 104 b (see FIG. 1F). Note that the channel length L ofthe transistor is determined by the distance between the edges of thesource electrode 104 a and the drain electrode 104 b which are formedhere.

As the conductive film used for the source electrode layer 104 a and thedrain electrode layer 104 b, a metal film containing an element selectedfrom Al, Cr, Cu, Ta, Ti, Mo, and W, a metal nitride film containing anyof the above elements as its component (e.g., a titanium nitride film, amolybdenum nitride film, or a tungsten nitride film), or the like may beused. Alternatively, a conductive film may be used in which ahigh-melting-point metal film of Ti, Mo, W, or the like or a metalnitride film of any of these elements (a titanium nitride film, amolybdenum nitride film, or a tungsten nitride film) may be stacked onone of or both a bottom side and a top side of a metal film of Al, Cu,or the like.

An etching step may be performed with the use of a resist mask formedusing a so-called multi-tone mask. A resist mask formed using amulti-tone mask has a plurality of thicknesses and can be furtherchanged in shape by ashing; thus, such a resist mask can be used in aplurality of etching steps for different patterns. Therefore, a resistmask corresponding to at least two kinds of different patterns can beformed by using one multi-tone mask. In other words, simplification ofthe steps can be realized.

Next, an insulating film 110 a in contact with part of the second oxidesemiconductor film 108 b and covering the source electrode layer 104 aand the drain electrode layer 104 b, and an insulating film 110 bcovering the source electrode layer 104 a and the drain electrode layer104 b are formed (see FIG. 1F).

It is preferable to use an insulating material containing a componentsimilar to that of the second oxide semiconductor film 108 b for theinsulating film 110 a. Such a material is compatible with the oxidesemiconductor film; thus, when it is used for the insulating film 110 a,the state of the interface between the insulating film 110 a and thesecond oxide semiconductor film 108 b can be kept favorably. Here, “acomponent similar to that of the second oxide semiconductor film 108 b”means one or more of elements selected from constituent metal elementsof the second oxide semiconductor film 108 b. For example, in the casewhere the second oxide semiconductor film 108 b is formed using anIn—Ga—Zn—O-based oxide semiconductor material, a gallium oxide or thelike is given as such an insulating material containing a componentsimilar to that of the second oxide semiconductor film 108 b.

Further, the insulating film 110 a preferably contains oxygen thatexceeds the stoichiometric proportion, more preferably contains oxygenmore than 1 time and less than two times the stoichiometric proportion.When the insulating film 110 a thus contains excessive oxygen, oxygen issupplied to the second oxide semiconductor film 108 b and the interfacebetween the insulating film 110 a and the second oxide semiconductorfilm 108 b, so that oxygen deficiency can be reduced.

The insulating film 110 b can be formed in a similar manner to the gateinsulating layer 102. That is, the insulating film 110 b can be formedto have a single-layer structure or a stacked-layer structure using amaterial such as silicon oxide, silicon nitride, silicon oxynitride,silicon nitride oxide, aluminum oxide, aluminum nitride, aluminumoxynitride, aluminum nitride oxide, or hafnium oxide, or a mixedmaterial thereof.

The insulating film 110 b preferably contains oxygen that exceeds thestoichiometric proportion, more preferably contains oxygen more than 1time and less than two times the stoichiometric proportion. When theinsulating film 110 b thus contains excessive oxygen, oxygen is suppliedto the second oxide semiconductor film 108 b and the interface betweenthe insulating film 110 a and the second oxide semiconductor film 108 b,so that oxygen deficiency can be reduced.

Through the above process, a transistor 120 is formed.

The transistor 120 shown in FIG. 1F includes the gate electrode layer112, the gate insulating layer 102, the first oxide semiconductor film108 a, the second oxide semiconductor film 108 b, the source electrodelayer 104 a, the drain electrode layer 104 b, the insulating film 110 a,and the insulating film 110 b, which are formed over the substrate 100.

In the transistor 120 shown in FIG. 1F, halogen doping treatment isperformed on the gate insulating layer 102. The c-axis of each of thefirst oxide semiconductor film 108 a and the second oxide semiconductorfilm 108 b are aligned perpendicularly to the surface of the crystallayer in the second oxide semiconductor film 108 b at least part ofwhich is crystallized. Consequently, the transistor 120 with higherreliability is realized.

In the transistor 120, the electronegativity of halogen such as chlorineor fluorine is larger than that of a metal (Zn, Ga, and In) in thestacked layer of the oxide semiconductor films, so that a hydrogen atomcan be taken away from a M-H bond in the stacked layer of the oxidesemiconductor films by containing halogen in the gate insulating layer102. Thus, a hydrogen ion detached from the M-H bond in the stackedlayer of the oxide semiconductor films which causes deterioration of thetransistor 120 can be trapped by halogen such as chlorine or fluorineadded at the interface between the stacked layer of the oxidesemiconductor films and the gate insulating layer 102.

(Embodiment 2)

In this embodiment, an example of steps which are partially differentfrom that described in Embodiment 1 will be described with reference toFIGS. 2A to 2F. Note that in FIGS. 2A to 2F, the same reference numeralsare used for the same parts in FIGS. 1A to 1F.

First, a conductive film is formed over the substrate 100 having aninsulating surface, and then the gate electrode layer 112 is formedthrough a photolithography step in a manner similar to that ofEmbodiment 1. The gate insulating layer 102 is formed over the gateelectrode layer 112 (see FIG. 2A).

If necessary, halogen doping treatment may be performed after the gateinsulating layer 102 is formed as in Embodiment 1. A halogen element maybe contained in the gate insulating layer 102 at the time of thedeposition. In that case, for example, a silicon oxide film containingfluorine is formed by a plasma CVD method with the use oftetrafluorosilane and oxygen and is used as the gate insulating layer102. Alternatively, the gate insulating layer 102 may be formed after adeposition chamber is cleaned with the use of a fluoride gas such asClF₃ or NF₃ so that fluorine or chlorine absorbed on inner walls of thedeposition chamber may be intentionally contained in the gate insulatinglayer 102.

Next, the first oxide semiconductor film 108 a with a thickness ofgreater than or equal to 2 nm and less than or equal to 15 nm or less isformed over the gate insulating layer 102 so as to overlap with the gateelectrode layer 112 (see FIG. 2B). Note that the first oxidesemiconductor film 108 a is a film which is subjected to first heattreatment for crystallization after the deposition of the oxidesemiconductor film and is crystallized as in Embodiment 1.

Next, the second oxide semiconductor film 108 b whose thickness is atleast greater than that of the first oxide semiconductor film 108 a andless than or equal to 10 μm is formed over the first oxide semiconductorfilm 108 a (see FIG. 2C). Note that the second oxide semiconductor film108 b is a film which is subjected to second heat treatment forcrystallization after the deposition of the oxide semiconductor film andis crystallized as in Embodiment 1.

Then, a stacked layer of the first oxide semiconductor film 108 a andthe second oxide semiconductor film 108 b is processed into anisland-shaped oxide semiconductor film (see FIG. 2D).

The stacked layer of the oxide semiconductor films can be processed byetching after a mask having a desired shape is formed over the stackedlayer of the oxide semiconductor films. The above mask can be formed bya method such as photolithography. Alternatively, a method such as aninkjet method may be used to form the mask.

For the etching of the stacked layer of the oxide semiconductor films,either wet etching or dry etching may be employed. It is needless to saythat these may be combined.

Note that the stacked layer of the oxide semiconductor films is notnecessarily processed to have island shape.

Next, the second oxide semiconductor film 108 b is subjected totreatment with oxygen 182 (also referred to as oxygen doping treatmentor oxygen plasma doping treatment) (see FIG. 2E). Here, at least any ofan oxygen radical, an oxygen atom, and an oxygen ion is included in theoxygen 182. By performing oxygen doping treatment on the stacked layerof the oxide semiconductor films, the oxygen can be contained in thestacked layer of the oxide semiconductor films, in the vicinity of theinterface of the stacked layer of the oxide semiconductor films, or inthe stacked layer of the oxide semiconductor films and the vicinity ofthe interface of the stacked layer of the oxide semiconductor films. Inthis case, the oxygen content is higher than the stoichiometricproportion of the stacked layer of the oxide semiconductor films,preferably greater than the stoichiometric proportion and less thandouble of the stoichiometric proportion. Alternatively, the oxygencontent may be greater than Y, preferably greater than Y and less than2Y, where the oxygen amount in the case where the material of thestacked layer of the oxide semiconductor films is a single crystal is Y.Still alternatively, the oxygen content may be greater than Z,preferably greater than Z and less than 2Z based on the oxygen amount Zin the insulating film in the case where oxygen doping is not performed.The reason why the above preferable range has the upper limit is thatthe stacked layer of the oxide semiconductor films might absorb hydrogenlike a hydrogen-storing alloy when the oxygen content is too high. Theoxygen content is greater than the hydrogen content in the stacked layerof the oxide semiconductor films.

In the case of a material expressed by the chemical formulaInGaO₃(ZnO)_(m) (m>0), x in InGaZnO_(x) can be greater than 4 and lessthan 8 when the crystalline structure where m is 1 (InGaZnO₄) is used asthe reference, and x in InGaZn₂O_(x) can be greater than 5 and less than10 when the crystalline structure where m is 2 (InGaZn₂O₅) is used asthe reference. Such an excessive oxygen region may exist in part of thestacked layer of the oxide semiconductor films (including theinterface).

Oxygen is one of the main component materials of an oxide semiconductorfilm. Thus, it is difficult to accurately estimate the oxygenconcentration of the oxide semiconductor film by a method such assecondary ion mass spectrometry (SIMS). In other words, it can be saidthat it is hard to determine whether oxygen is intentionally added tothe oxide semiconductor film.

Isotopes such as ¹⁷O and ¹⁸O exist in oxygen, and it is know that theexistence proportions of them in nature are about 0.037% and about0.204% of all the oxygen atoms. That is to say, it is possible tomeasure the concentrations of these isotopes in the oxide semiconductorfilm by a method such as SIMS; therefore, the oxygen concentration ofthe oxide semiconductor film may be able to be estimated more accuratelyby measuring the concentrations of these isotopes. Thus, theconcentrations of these isotopes may be measured to determine whetheroxygen is intentionally added to the oxide semiconductor film.

For example, when the concentration of ¹⁸O is used as the reference, D1(¹⁸O)>D2 (¹⁸O) is satisfied where D1 (¹⁸O) is the concentration of anisotope of oxygen in a region of the oxide semiconductor film to whichoxygen has been added, and D2 (¹⁸O) is the concentration of an isotopeof oxygen in a region of the oxide semiconductor film to which oxygen isnot added.

It is preferable that at least part of the oxygen 182 added to thestacked layer of the oxide semiconductor films have dangling bonds inthe stacked layer of the oxide semiconductor films. This is because,with the dangling bond, the oxygen 182 can be bonded with hydrogen whichcan remain in the film, so that the hydrogen can be fixed (made to be animmovable ion).

The oxygen 182 can be generated by a plasma generating apparatus or anozone generating apparatus. Specifically, for example, the oxygen 182 isgenerated using an apparatus capable of performing etching on asemiconductor device, an apparatus capable of performing ashing on aresist mask, or the like, and the stacked layer of the oxidesemiconductor films can be processed. Alternatively, the oxygen may beadded by performing irradiation with an oxygen ion accelerated by anelectric field. A high-density plasma CVD method using microwaves (forexample, a frequency of 2.45 GHz) may be employed to add the oxygen. Inthe case where a high-density plasma CVD method using microwaves isemployed, the stacked layer of oxide semiconductor films is hardlydamaged at the addition of the oxygen.

In order to add oxygen more preferably, it is preferable to apply anelectrical bias to a substrate when treatment with oxygen is performed.For example, in the case where the oxygen is added using an ICPapparatus, high frequency power supply of greater than or equal to 1 kWand less than or equal to 10 kW may be applied to an ICP coil and asubstrate bias of 1000 W may be applied to a substrate stage.

Note that heat treatment may be performed on the stacked layer of theoxide semiconductor films being subjected to the oxygen dopingtreatment. The heat treatment temperature is higher than or equal to250° C. and lower than or equal to 700° C., preferably higher than orequal to 400° C. and lower than or equal to 600° C., or lower than thestrain point of the substrate.

Through the heat treatment, water, a hydroxide (OH), and the likegenerated by reaction between oxygen and hydrogen contained in thematerial of the oxide semiconductor can be removed from the oxidesemiconductor film. Hydrogen or the like entered the stacked layer ofthe oxide semiconductor films or the like due to the above oxygen dopingtreatment can also be removed by this heat treatment. The heat treatmentmay be performed in an atmosphere from which water, hydrogen, or thelike is satisfactorily reduced, such as a nitrogen atmosphere, an oxygenatmosphere, an ultra-dry air atmosphere (the moisture amount is lessthan or equal to 20 ppm (−55° C. by conversion into a dew point),preferably less than or equal to 1 ppm, more preferably less than orequal to 10 ppb when measured with the use of a dew point meter of acavity ring down laser spectroscopy (CRDS) system), or a rare gasatmosphere (such as argon and helium). In particular, the heat treatmentis preferably performed in an atmosphere containing oxygen. Further, thepurity of nitrogen, oxygen, or a rare gas introduced into a heattreatment apparatus is preferably higher than or equal to 6N (99.9999%)(that is, lower than or equal to the impurity concentration is 1 ppm),further preferably higher than or equal to 7N (99.99999%) (that is,lower than or equal to the impurity concentration is 0.1 ppm).

The oxygen doping treatment and the heat treatment may be repeated. Byrepeatedly performing the oxygen doping treatment and the heattreatment, the transistor can have higher reliability. Note that thenumber of repetitions can be set appropriately.

In the heat treatment according to this embodiment, the stacked layer ofthe oxide semiconductor films is desirably heated in an atmospherecontaining oxygen. Thus, oxygen, which may be reduced due to the abovecrystallization treatment, can be supplied to the stacked layer of theoxide semiconductor films. In this sense, the heat treatment can also bereferred to as supply of oxygen. The crystallization treatment is alsoreferred to as dehydration treatment (or dehydrogenation treatment) bywhich impurities containing hydrogen in the oxide semiconductor (such aswater and OH) are eliminated.

Note that there is no particular limitation on the timing of the heattreatment for supply of oxygen as long as it is after formation of thestacked layer of the oxide semiconductor films. For example, the heattreatment for supply of oxygen may be performed after the formation ofthe insulating film 110 a described later. Alternatively, the heattreatment for supply of oxygen may be performed after the gate electrodelayer is formed. The heat treatment for supply of oxygen may beperformed following to heat treatment for dehydration or the like; heattreatment for dehydration or the like may also serve as the heattreatment supply of oxygen; the heat treatment for supply of oxygen mayalso serve as heat treatment for dehydration or the like.

As described above, when heat treatment for dehydration or the like andoxygen doping treatment or heat treatment for supplying oxygen areperformed, the stacked layer of the oxide semiconductor films can behighly purified so as to contain impurities except main components ofthe stacked layer of oxide semiconductor films as little as possible.The stacked layer of the oxide semiconductor films which is highlypurified contains extremely few (close to zero) carriers derived from adonor.

Note that in this embodiment, the heat treatment for crystallization orthe like is performed, the stacked layer of the oxide semiconductorfilms is processed to have an island shape, the oxygen doping treatmentis performed, and the heat treatment for supply of oxygen is performed;however, these steps are not limited to this order.

Next, a conductive film for forming a source electrode layer and a drainelectrode layer (including a wiring formed in the same layer as thesource electrode and the drain electrode) is formed over the gateinsulating layer 102 and the stacked layer of the oxide semiconductorfilms and processed to form the source electrode layer 104 a and thedrain electrode layer 104 b in a manner similar to that of Embodiment 1(see FIG. 2F).

Next, the insulating film 110 a being in contact with part of the secondoxide semiconductor film 108 b and covering the source electrode layer104 a and the drain electrode layer 104 b, and the insulating film 110 bcovering the source electrode 104 a and the drain electrode 104 b areformed (see FIG. 2F).

Through the above process, a transistor 130 is formed. In the transistor130, the stacked layer of the oxide semiconductor films is crystallizedand the oxygen content of the oxide semiconductor film is increased;thus, deterioration due to electrical bias stress or heat stress can besuppressed and deterioration due to light can be reduced.

Note that the above description is an example of performing oxygendoping treatment on the stacked layer of the oxide semiconductor filmswhich is processed to have an island shape; however, one embodiment ofthe disclosed invention is not limited to this. For example, the stackedlayer of the oxide semiconductor films may be processed to have anisland shape after crystallization treatment and oxygen doping treatmentare performed, or oxygen doping treatment may be performed afterformation of the source electrode layer 104 a and the drain electrodelayer 104 b.

(Embodiment 3)

In this embodiment, cross-sectional views of a transistor 140 and atransistor 150 are described as modified examples of the transistor 120and the transistor 130 shown in FIG. 1F and FIG. 2F respectively.

The transistor 140 shown in FIG. 3A is the same as the transistor 120 inthat it includes the gate electrode layer 112, the gate insulating layer102, the first oxide semiconductor film 108 a, the second oxidesemiconductor film 108 b, the source electrode layer 104 a, the drainelectrode layer 104 b, the insulating film 110 a, and the insulatingfilm 110 b, which are formed over the substrate 100. The differencebetween the transistor 140 and the transistor 120 is the presence of asecond gate insulating layer 103 and an insulating film 114 covering theabove components. In other words, the transistor 140 includes the secondgate insulating layer 103 and the insulating film 114. The othercomponents are the same as those of the transistor 120 in FIG. 1F; thus,the description of FIGS. 1A to 1F can be referred to for the detailsthereof.

It is particularly preferable to use an insulating material containing acomponent similar to that of the first oxide semiconductor film 108 afor the second gate insulating layer 103. Such a material is compatiblewith the first oxide semiconductor film 108 a; thus, when it is used forthe second gate insulating layer 103, the state of the interface betweenthe first oxide semiconductor film 108 a and the second gate insulatinglayer 103 can be kept favorably. Here, “a component similar to that ofthe first oxide semiconductor film 108 a” means one or more of elementsselected from constituent metal elements of the first oxidesemiconductor film 108 a. In the case where the first oxidesemiconductor film 108 a is formed of an In—Ga—Zn-based oxidesemiconductor material, gallium oxide or the like is given as theinsulating material containing a component similar to that of the firstoxide semiconductor film 108 a, for example.

Note that the insulating film 114 can be formed using silicon oxide,silicon nitride, aluminum oxide, aluminum nitride, or gallium oxide; amixed material thereof; or the like. In particular, a silicon nitridefilm is preferable as the insulating film 114 because added oxygen canbe prevented from being released to the outside and entry of hydrogenand the like to the stacked layer of the oxide semiconductor films fromthe outside can be suppressed effectively. The hydrogen content of theinsulating film 114 is less than or equal to 1/10 of that of thenitrogen content thereof, and is preferably less than 1×10²⁰ cm⁻³, morepreferably less than 5×10¹⁸ cm⁻³. Note that a wiring connected to thesource electrode layer 104 a, the drain electrode layer 104 b, the gateelectrode layer 112, and the like may be formed over the insulating film114.

The transistor 150 shown in FIG. 3B is the same as the transistor 120 inthat it includes the gate electrode layer 112, the gate insulating layer102, the first oxide semiconductor film 108 a, the second oxidesemiconductor film 108 b, the source electrode layer 104 a, the drainelectrode layer 104 b, the insulating film 110 a, and the insulatingfilm 110 b, which are formed over the substrate 100. The differencebetween the transistor 150 and the transistor 120 is the presence of asecond gate electrode layer 105. That is, the transistor 150 includesthe second gate electrode layer 105. The other components are the sameas those of the transistor 120 in FIG. 1F; thus, the description ofFIGS. 1A to 1F can be referred to for the details thereof.

Note that the second gate electrode layer 105 is an electrode layerwhich can function as a back gate. The potential of the back gate can bea fixed potential, e.g., 0 V, or a ground potential, and may bedetermined as appropriate by practitioners. In addition, by providingthe gate electrode layers above and below the stacked layer of thesemiconductor films in the transistor 150, the amount of shift inthreshold voltage of the transistor can be reduced in a BT test forexamining the reliability of the transistor. That is, by providing thegate electrodes above and below the stacked layer of the oxidesemiconductor films, the reliability can improve. Further, bycontrolling gate voltage applied to the back gate, the threshold voltagecan be controlled. Furthermore, when a light-blocking conductive film isused for the second gate electrode layer 105, the second gate electrode105 can function as a light-blocking film of the stacked layer of theoxide semiconductor films, so that the reliability can be improved.

This embodiment can be freely combined with Embodiment 1 or 2.

(Embodiment 4)

A semiconductor device having a display function (also referred to as adisplay device) can be manufactured using the transistor exemplified inany of Embodiments 1 to 3. Moreover, some or all of driver circuitswhich include the transistor can be formed over a substrate where apixel portion is formed, whereby a system-on-panel can be obtained.

In FIG. 4A, a sealant 4005 is provided so as to surround a pixel portion4002 provided over a first substrate 4001, and the pixel portion 4002 issealed between the first substrate 4001 and a second substrate 4006. InFIG. 4A, a signal line driver circuit 4003 and a scan line drivercircuit 4004 which are formed using a single crystal semiconductor filmor a polycrystalline semiconductor film over a substrate separatelyprepared are mounted in a region that is different from the regionsurrounded by the sealant 4005 over the first substrate 4001. Varioussignals and potential are supplied to the signal line driver circuit4003 and the scan line driver circuit 4004 each of which is separatelyformed, and the pixel portion 4002 from flexible printed circuits (FPCs)4018 a and 4018 b.

In FIGS. 4B and 4C, the sealant 4005 is provided so as to surround thepixel portion 4002 and the scan line driver circuit 4004 which areprovided over the first substrate 4001. The second substrate 4006 isprovided over the pixel portion 4002 and the scan line driver circuit4004. Consequently, the pixel portion 4002 and the scan line drivercircuit 4004 are sealed together with a display element, by the firstsubstrate 4001, the sealant 4005, and the second substrate 4006. InFIGS. 4B and 4C, the signal line driver circuit 4003 which is formedusing a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate prepared separately is mounted in aregion that is different from the region surrounded by the sealant 4005over the first substrate 4001. In FIGS. 4B and 4C, a variety of signalsand potentials are supplied to the signal line driver circuit 4003 whichis separately formed, and the scan line driver circuit 4004 or the pixelportion 4002 from an FPC 4018.

Although FIGS. 4B and 4C each illustrate an example in which the signalline driver circuit 4003 is formed separately and mounted on the firstsubstrate 4001, the present invention is not limited to this structure.The scan line driver circuit may be separately formed and then mounted,or only part of the signal line driver circuit or part of the scan linedriver circuit may be separately formed and then mounted.

Note that a connection method of a separately formed driver circuit isnot particularly limited, and a chip on glass (COG) method, a wirebonding method, a tape automated bonding (TAB) method or the like can beused. FIG. 4A shows an example in which the signal line driver circuit4003 and the scan line driver circuit 4004 are mounted by a COG method.FIG. 4B shows an example in which the signal line driver circuit 4003 ismounted by a COG method. FIG. 4C shows an example in which the signalline driver circuit 4003 is mounted by a TAB method.

In addition, the display device includes a panel in which the displayelement is sealed, and a module in which an IC or the like including acontroller is mounted on the panel.

Note that a display device in this specification means an image displaydevice, a display device, or a light source (including a lightingdevice). Furthermore, the display device also includes the followingmodules in its category: a module to which a connector such as an FPC, aTAB tape, or a TCP is attached; a module having a TAB tape or a TCP atthe tip of which a printed wiring board is provided; and a module inwhich an integrated circuit (IC) is directly mounted on a displayelement by a COG method.

The pixel portion and the scan line driver circuit provided over thefirst substrate include a plurality of transistors; any of thetransistors which are described in Embodiments 1 to 3 can be appliedthereto.

A liquid crystal element (also referred to as a liquid crystal displayelement) or a light-emitting element (also referred to as alight-emitting display element) can be used as the display elementprovided in the display device. The light-emitting element includes, inits category, an element whose luminance is controlled by a current or avoltage, and specifically includes, in its category, an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Furthermore, a display medium whose contrast is changed by an electriceffect, such as electronic ink, can be used.

Embodiments of the semiconductor device will be described with referenceto FIG. 5 and FIG. 6. FIG. 5 and FIG. 6 correspond to cross-sectionalviews along line M-N in FIG. 4B.

As shown in FIG. 5 and FIG. 6, the semiconductor device includes aconnection terminal electrode 4015 and a terminal electrode 4016, andthe connection terminal electrode 4015 and the terminal electrode 4016are electrically connected to a terminal included in the FPC 4018through an anisotropic conductive film 4019.

The connection terminal electrode 4015 is formed using the sameconductive film as a first electrode layer 4030, and the terminalelectrode 4016 is formed using the same conductive film as source anddrain electrode layers of a transistor 4010 and a transistor 4011.

The pixel portion 4002 and the scan line driver circuit 4004 providedover the first substrate 4001 include a plurality of transistors. FIG. 5and FIG. 6 each illustrate the transistor 4010 included in the pixelportion 4002 and the transistor 4011 included in the scan line drivercircuit 4004, as an example. In FIG. 6, an insulating layer 4021 isprovided over the transistors 4010 and 4011.

In this embodiment, the transistors described in any one of Embodiments1 to 3 can be applied to the transistor 4010 and the transistor 4011.Variation in electrical characteristics of the transistor 4010 and thetransistor 4011 is suppressed and the transistor 4010 and the transistor4011 are electrically stable. Consequently, semiconductor devices withhigh reliability can be provided as the semiconductor devices of thisembodiment shown in FIG. 5 and FIG. 6.

In addition, in this embodiment, a conductive layer may be provided in aregion over an insulating layer overlapping with a channel formationregion of the oxide semiconductor film in the transistor 4011 for thedriver circuit. By providing the conductive layer so as to overlap withthe channel formation region of the oxide semiconductor film, the amountof change in the threshold voltage of the transistor 4011 by the BT testcan be further reduced. The potential of the conductive layer may be thesame as or different from that of a gate electrode layer of thetransistor 4011, and the conductive layer can be functioned as a secondgate electrode layer. The potential of the conductive layer may be GND,0V, or in a floating state.

The conductive layer also functions to block an external electric field,that is, to prevent an external electric field (particularly, to preventstatic electricity) from effecting the inside (a circuit portionincluding a transistor). A blocking function of the conductive layer canprevent the variation in electrical characteristics of the transistordue to the effect of external electric field such as static electricity.

The transistor 4010 included in the pixel portion 4002 is electricallyconnected to a display element to form a display panel. There is noparticular limitation on the kind of the display element as long asdisplay can be performed, and various kinds of display elements can beemployed.

An example of a liquid crystal display device using a liquid crystalelement as a display element is illustrated in FIG. 5. In FIG. 5, aliquid crystal element 4013 which is a display element includes thefirst electrode layer 4030, a second electrode layer 4031, and a liquidcrystal layer 4008. Insulating films 4032 and 4033 serving as alignmentfilms are provided so that the liquid crystal layer 4008 is interposedtherebetween. The second electrode layer 4031 is provided on the secondsubstrate 4006 side, and the first electrode layer 4030 and the secondelectrode layer 4031 are stacked, with the liquid crystal layer 4008interposed therebetween.

A columnar spacer denoted by reference numeral 4035 is obtained byselective etching of an insulating film and is provided in order tocontrol the thickness (a cell gap) of the liquid crystal layer 4008.Note that the spacer 4035 is not limited to a columnar spacer, and, forexample, a spherical spacer may be used.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material exhibits acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on a condition.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while temperature of cholesteric liquidcrystal is increased. Since the blue phase appears only in a narrowtemperature range, a liquid crystal composition in which 5 wt. % or moreof a chiral material is mixed is used for the liquid crystal layer inorder to improve the temperature range. The liquid crystal compositionwhich includes a liquid crystal showing a blue phase and a chiralmaterial has a short response time of 1 msec or less, has opticalisotropy, which makes the alignment process unneeded, and has a smallviewing angle dependence. In addition, since an alignment film does notneed to be provided and rubbing treatment is unnecessary, electrostaticdischarge damage caused by the rubbing treatment can be prevented anddefects and damage of the liquid crystal display device can be reducedin the manufacturing process. Thus, productivity of the liquid crystaldisplay device can be increased.

The specific resistivity of the liquid crystal material is 1×10⁹ Ω·cm ormore, preferably 1×10¹¹ Ω·cm or more, more preferably 1×10¹² Ω·cm ormore. The value of the specific resistivity in this specification ismeasured at 20° C.

The size of storage capacitor formed in the liquid crystal displaydevice is set considering the leakage current of the transistor providedin the pixel portion or the like so that charge can be held for apredetermined period. By using the transistor including the highlypurified oxide semiconductor film, it is enough to provide a storagecapacitor having a capacitance that is ⅓ or less, preferably ⅕ or lessof a liquid crystal capacitance of each pixel.

Further, the current value in an off state (off state current value) ofthe transistor used in this embodiment can be made small. Therefore, anelectrical signal such as an image signal can be held for a longerperiod in the pixel, and a writing interval can be set longer in an onstate. Consequently, frequency of refresh operation can be reduced,which leads to an effect of suppressing power consumption.

In addition, the transistor used in this embodiment can have relativelyhigh field-effect mobility and thus is capable of high speed operation.Therefore, by using the transistor in a pixel portion of a liquidcrystal display device, a high-quality image can be provided. A drivercircuit portion and a pixel portion each of which include the transistorcan be formed over one substrate; thus, the number of components of thesemiconductor device can be reduced.

For the liquid crystal display device, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, anaxially symmetric aligned micro-cell (ASM) mode, an optical compensatedbirefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, anantiferroelectric liquid crystal (AFLC) mode, or the like can be used.

A normally black liquid crystal display device such as a transmissiveliquid crystal display device utilizing a vertical alignment (VA) modeis preferable. Here, the vertical alignment mode is a method ofcontrolling alignment of liquid crystal molecules of a liquid crystaldisplay panel, in which liquid crystal molecules are aligned verticallyto a panel surface when no voltage is applied. Some examples are givenas the vertical alignment mode. For example, a multi-domain verticalalignment (MVA) mode, a patterned vertical alignment (PVA) mode, an ASVmode, and the like can be used. Moreover, it is possible to use a methodcalled domain multiplication or multi-domain design, in which a pixel isdivided into some regions (subpixels) and molecules are aligned indifferent directions in their respective regions.

In the display device, a black matrix (a light-blocking layer), anoptical member (an optical substrate) such as a polarizing member, aretardation member, or an anti-reflection member, and the like areprovided as appropriate. For example, circular polarization may beobtained by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

In addition, it is possible to employ a time-division display method(also called a field-sequential driving method) with the use of aplurality of light-emitting diodes (LEDs) as a backlight. By employing afield-sequential driving method, color display can be performed withoutusing a color filter.

As a display method in the pixel portion, a progressive method, aninterlace method or the like can be employed. Further, color elementscontrolled in a pixel at the time of color display are not limited tothree colors: R, G, and B (R, G, and B correspond to red, green, andblue respectively). For example, R, G, B, and W (W corresponds towhite); R, G, B, and one or more of yellow, cyan, magenta, and the like;or the like can be used. Further, the sizes of display regions may bedifferent between respective dots of color elements.

Alternatively, as the display element included in the display device, alight-emitting element utilizing electroluminescence can be used.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. The carriers (electrons and holes) are recombined,and thus, the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

In order to extract light emitted from the light-emitting element, it isacceptable as long as at least one of a pair of electrodes istransparent. The light-emitting element can have a top emissionstructure in which light emission is extracted through the surfaceopposite to the substrate; a bottom emission structure in which lightemission is extracted through the surface on the substrate side; or adual emission structure in which light emission is extracted through thesurface opposite to the substrate and the surface on the substrate side,and a light-emitting element having any of these emission structures canbe used.

Further, an electronic paper in which electronic ink is driven can beprovided as the display device. The electronic paper is also called anelectrophoretic display device (electrophoretic display) and hasadvantages in that it has the same level of readability as regularpaper, it has less power consumption than other display devices, and itcan be set to have a thin and light form.

An electrophoretic display device can have various modes. Anelectrophoretic display device contains a plurality of microcapsulesdispersed in a solvent or a solute, each microcapsule containing firstparticles which are positively charged and second particles which arenegatively charged. By applying an electric field to the microcapsules,the particles in the microcapsules move in opposite directions to eachother and only the color of the particles gathering on one side isdisplayed. Note that the first particles or the second particles containpigment and do not move without an electric field. Moreover, the firstparticles and the second particles have different colors (which may becolorless).

Thus, an electrophoretic display device is a display device thatutilizes a so-called dielectrophoretic effect by which a substancehaving a high dielectric constant moves to a high-electric field region.

A solution in which the above microcapsules are dispersed in a solventis referred to as electronic ink. This electronic ink can be printed ona surface of glass, plastic, cloth, paper, or the like. Furthermore, byusing a color filter or particles that have a pigment, color display canalso be achieved.

Note that the first particles and the second particles in themicrocapsules may each be formed of a single material selected from aconductive material, an insulating material, a semiconductor material, amagnetic material, a liquid crystal material, a ferroelectric material,an electroluminescent material, an electrochromic material, and amagnetophoretic material, or formed of a composite material of any ofthese.

As the electronic paper, a display device using a twisting ball displaysystem can be used. The twisting ball display system refers to a methodin which spherical particles each colored in black and white arearranged between a first electrode layer and a second electrode layerwhich are electrode layers used for a display element, and a potentialdifference is generated between the first electrode layer and the secondelectrode layer to control orientation of the spherical particles, sothat display is performed.

FIG. 6 shows active matrix electronic paper as one embodiment of asemiconductor device. The electronic paper in FIG. 6 is an example of adisplay device using a twisting ball display system.

Between the first electrode layer 4030 connected to the transistor 4010and the second electrode layer 4031 provided on the second substrate4006, spherical particles 4613 each of which includes a black region4615 a, a white region 4615 b, and a cavity 4612 which is filled withliquid around the black region 4615 a and the white region 4615 b, areprovided. A space around the spherical particles 4613 is filled with afiller 4614 such as a resin. The second electrode layer 4031 correspondsto a common electrode (counter electrode). The second electrode layer4031 is electrically connected to a common potential line.

In FIG. 5 and FIG. 6, as the first substrate 4001 and the secondsubstrate 4006, flexible substrates, for example, plastic substrateshaving a light-transmitting property or the like can be used, inaddition to glass substrates. As plastic, a fiberglass-reinforcedplastics (FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film,or an acrylic resin film can be used. In addition, a sheet with astructure in which an aluminum foil is sandwiched between PVF films orpolyester films can be used.

The insulating layer 4021 can be formed using an inorganic insulatingmaterial or an organic insulating material. Note that the insulatinglayer 4021 formed using a heat-resistant organic insulating materialsuch as an acrylic resin, polyimide, a benzocyclobutene-based resin,polyamide, or an epoxy resin is preferably used as a planarizinginsulating film. Other than such organic insulating materials, it ispossible to use a low-dielectric constant material (a low-k material), asiloxane based resin, phosphosilicate glass (PSG), borophosphosilicateglass (BPSG), or the like. The insulating layer may be formed bystacking a plurality of insulating films formed of these materials.

There is no particular limitation on the method for forming theinsulating layer 4021, and the insulating layer 4021 can be formed,depending on the material, by a sputtering method, a spin coatingmethod, a dipping method, spray coating, a droplet discharge method(e.g., an inkjet method, screen printing, or offset printing), or thelike.

The display device displays images by transmitting light from a lightsource or a display element. Therefore, the substrate and the thin filmssuch as the insulating film and the conductive film provided for thepixel portion where light is transmitted have light-transmittingproperties with respect to light in the visible-light wavelength range.

The first electrode layer 4030 and the second electrode layer 4031 (eachof which may be called a pixel electrode layer, a common electrodelayer, a counter electrode layer, or the like) for applying voltage tothe display element may have light-transmitting properties orlight-reflecting properties, which depends on the direction in whichlight is extracted, the position where the electrode layer is provided,the pattern structure of the electrode layer, and the like.

The first electrode layer 4030 and the second electrode layer 4031 canbe formed using a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (hereinafter referred to asITO), indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

The first electrode layer 4030 and the second electrode layer 4031 canbe formed of one or more kinds of materials selected from metals such astungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium(V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel(Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), andsilver (Ag); alloys of these metals; and nitrides of these metals.

Since the transistor is easily broken owing to static electricity or thelike, a protective circuit for protecting the driver circuit ispreferably provided. The protective circuit is preferably formed using anonlinear element.

As described above, by using the transistors exemplified in any one ofEmbodiments 1 to 3, a highly reliable semiconductor device can beprovided.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

(Embodiment 5)

A semiconductor device disclosed in this specification can be applied toa variety of electronic appliances (including game machines). Examplesof electronic appliances are a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game machine, apersonal digital assistant, an audio reproducing device, a large-sizedgame machine such as a pachinko machine, and the like. Examples ofelectronic appliances each including the display device described in anyof the above embodiments will be described.

FIG. 7A shows an electronic book reader (also referred to as an e-bookreader) which can include housings 9630, a display portion 9631,operation keys 9632, a solar battery 9633, and a charge and dischargecontrol circuit 9634. The electronic book reader shown in FIG. 7A has afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image) on the display portion, a function ofdisplaying a calendar, a date, the time, or the like on the displayportion, a function of operating or editing the data displayed on thedisplay portion, a function of controlling processing by various kindsof software (programs), and the like. Note that in FIG. 7A, the chargeand discharge control circuit 9634 has a battery 9635 and a DCDCconverter (hereinafter abbreviated as a converter) 9636 as an example.The semiconductor device described in Embodiment 4 can be applied to thedisplay portion 9631, whereby a highly reliable electronic book readercan be provided.

In the case of using a transflective or reflective liquid crystaldisplay device as the display portion 9631 in the structure shown inFIG. 7A, the electronic book reader may be used in a comparativelybright environment. In that case, power generation by the solar battery9633 and charge by the battery 9635 can be effectively performed, whichis preferable. Since the solar battery 9633 can be provided on a space(a surface or a rear surface) of the housings 9630 as appropriate, thebattery 9635 can be efficiently charged, which is preferable. When alithium ion battery is used as the battery 9635, there is an advantageof downsizing or the like.

The structure and the operation of the charge and discharge controlcircuit 9634 shown in FIG. 7A are described with reference to a blockdiagram in FIG. 7B. The solar battery 9633, the battery 9635, theconverter 9636, a converter 9637, switches SW1 to SW3, and the displayportion 9631 are shown in FIG. 7B, and the battery 9635, the converter9636, the converter 9637, and the switches SW1 to SW3 correspond to thecharge and discharge control circuit 9634.

First, an example of operation in the case where power is generated bythe solar battery 9633 using external light is described. The voltage ofpower generated by the solar battery 9633 is raised or lowered by theconverter 9636 so that the power has a voltage for charging the battery9635. When the power from the solar battery 9633 is used for theoperation of the display portion 9631, the switch SW1 is turned on andthe voltage of the power is raised or lowered by the converter 9637 soas to be a voltage needed for the display portion 9631. In addition,when display on the display portion 9631 is not performed, the switchSW1 is turned off and the switch SW2 is turned on so that the battery9635 may be charged.

Next, operation in the case where power is not generated by the solarbattery 9633 using external light is described. The voltage of poweraccumulated in the battery 9635 is raised or lowered by the converter9637 by turning on the switch SW3. Then, power from the battery 9635 isused for the operation of the display portion 9631.

Note that although the solar battery 9633 is described as an example ofa means for charge, the battery 9635 may be charged by another means. Inaddition, a combination of the solar battery 9633 and another means forcharge may be used.

FIG. 8A shows a laptop personal computer, which includes a main body3001, a housing 3002, a display portion 3003, a keyboard 3004, and thelike. The semiconductor device described in Embodiment 4 is applied tothe display portion 3003, whereby a highly reliable laptop personalcomputer can be provided.

FIG. 8B is a personal digital assistant (PDA) including a displayportion 3023, an external interface 3025, an operation button 3024, andthe like in a main body 3021. A stylus 3022 is included as an accessoryfor operation. The semiconductor device described in Embodiment 4 isapplied to the display portion 3023, whereby a highly reliable personaldigital assistant (PDA) can be provided.

FIG. 8C is an example of an e-book reader. For example, the e-bookreader 2700 includes two housings, a housing 2701 and a housing 2703.The housing 2701 and the housing 2703 are combined with a hinge 2711 sothat the e-book reader 2700 can be opened and closed with the hinge 2711as an axis. With such a structure, the e-book reader 2700 can operatelike a paper book.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703 respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the structure where different images are displayed indifferent display portions, for example, the right display portion (thedisplay portion 2705 in FIG. 8C) can display text and the left displayportion (the display portion 2707 in FIG. 8C) can display images. Whenthe semiconductor device shown in Embodiment 4 is applied to the displayportions 2705 and 2707, the e-book reader 2700 with high reliability canbe obtained.

FIG. 8C shows an example in which the housing 2701 is provided with anoperation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, operation keys 2723, a speaker 2725,and the like. With the operation keys 2723, pages can be turned. Notethat a keyboard, a pointing device, or the like may also be provided onthe surface of the housing, on which the display portion is provided.Furthermore, an external connection terminal (an earphone terminal, aUSB terminal, or the like), a recording medium insertion portion, andthe like may be provided on the back surface or the side surface of thehousing. Moreover, the e-book reader 2700 may have a function of anelectronic dictionary.

The e-book reader 2700 may have a structure capable of wirelesslytransmitting and receiving data. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an electronicbook server.

FIG. 8D is a mobile phone, which includes two housings, a housing 2800and a housing 2801. The housing 2801 includes a display panel 2802, aspeaker 2803, a microphone 2804, a pointing device 2806, a camera lens2807, an external connection terminal 2808, and the like. In addition,the housing 2800 includes a solar cell 2810 having a function of chargeof the mobile phone, an external memory slot 2811, and the like.Further, an antenna is incorporated in the housing 2801. Thesemiconductor device described in Embodiment 4 is applied to the displaypanel 2802, whereby a highly reliable mobile phone can be provided.

The display panel 2802 is provided with a touch panel. A plurality ofoperation keys 2805 which is displayed as images is illustrated bydashed lines in FIG. 8D. Note that a boosting circuit by which a voltageoutput from the solar cell 2810 is increased to be sufficiently high foreach circuit is also included.

The display direction of the display panel 2802 can be appropriatelychanged depending on a usage pattern. Further, the display device isprovided with the camera lens 2807 on the same surface as the displaypanel 2802, and thus it can be used as a video phone. The speaker 2803and the microphone 2804 can be used for videophone calls, recording andplaying sound, and the like as well as voice calls. Moreover, thehousing 2800 and the housing 2801 developed as shown in FIG. 8D can beslid so that one is lapped over the other; thus, the size of the mobilephone can be reduced, which makes the mobile phone suitable for beingcarried.

The external connection terminal 2808 can be connected to an AC adapterand various types of cables such as a USB cable, and charging and datacommunication with a personal computer are possible. Moreover, a largeamount of data can be stored by inserting a storage medium into theexternal memory slot 2811 and can be moved.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be provided.

FIG. 8E is a digital video camera which includes a main body 3051, adisplay portion A 3057, an eyepiece 3053, an operation switch 3054, adisplay portion B 3055, a battery 3056, and the like. The semiconductordevice described in Embodiment 4 is applied to the display portion A3057 and the display portion B 3055, whereby a highly reliable digitalvideo camera can be provided.

FIG. 8F is an example of a television device. In a television set 9600,a display portion 9603 is incorporated in a housing 9601. The displayportion 9603 can display images. Here, the housing 9601 is supported bya stand 9605. When the semiconductor device shown in Embodiment 4 isapplied to the display portion 9603, the television set 9600 with highreliability can be obtained.

The television set 9600 can be operated by an operation switch of thehousing 9601 or a separate remote controller. Further, the remotecontroller may be provided with a display portion for displaying datawhich is output from the remote controller.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the display device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

The structures and methods described in this embodiment can be combinedas appropriate with any of the structures and methods described in theother embodiments.

This application is based on Japanese Patent Application serial no.2010-117020 filed with Japan Patent Office on May 21, 2010, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing a semiconductordevice, comprising the steps of: forming a gate electrode layer over asubstrate; forming a gate insulating layer over the gate electrodelayer; forming a stacked oxide semiconductor film over the gateinsulating layer, the stacked oxide semiconductor film comprising afirst oxide semiconductor film and a second oxide semiconductor filmover the first oxide semiconductor film; performing oxygen dopingtreatment on the stacked oxide semiconductor film; and forming a sourceelectrode layer and a drain electrode layer over the stacked oxidesemiconductor film, wherein each of the first oxide semiconductor filmand the second oxide semiconductor film contains indium, wherein thefirst oxide semiconductor film includes crystals which are c-axisaligned perpendicularly to a surface of the first oxide semiconductorfilm, and wherein the second oxide semiconductor film includes crystalswhich are c-axis aligned perpendicularly to a surface of the secondoxide semiconductor film.
 2. The method for manufacturing asemiconductor device according to claim 1, further comprising a step ofperforming halogen doping treatment on the gate insulating layer beforeforming the first oxide semiconductor film.
 3. The method formanufacturing a semiconductor device according to claim 1, wherein athickness of the second oxide semiconductor film is larger than athickness of the first oxide semiconductor film.
 4. The method formanufacturing a semiconductor device, according to claim 1, furthercomprising a step of forming an oxide film over and in contact with thesource electrode layer, the drain electrode layer, and the second oxidesemiconductor film.
 5. The method for manufacturing a semiconductordevice, according to claim 4, wherein the oxide film includes one ormore elements selected from constituent metal element of the secondoxide semiconductor film.
 6. A method for manufacturing a semiconductordevice, comprising the steps of: forming a gate electrode layer over asubstrate; forming a gate insulating layer over the gate electrodelayer; forming a stacked oxide semiconductor film over the gateinsulating layer, the stacked oxide semiconductor film comprising afirst oxide semiconductor film and a second oxide semiconductor filmover the first oxide semiconductor film; performing oxygen dopingtreatment on the stacked oxide semiconductor film; performing a heattreatment on the stacked oxide semiconductor film; and forming a sourceelectrode layer and a drain electrode layer over the stacked oxidesemiconductor film, wherein each of the first oxide semiconductor filmand the second oxide semiconductor film contains indium, wherein thefirst oxide semiconductor film includes crystals which are c-axisaligned perpendicularly to a surface of the first oxide semiconductorfilm, and wherein the second oxide semiconductor film includes crystalswhich are c-axis aligned perpendicularly to a surface of the secondoxide semiconductor film.
 7. The method for manufacturing asemiconductor device according to claim 6 further comprising a step ofperforming halogen doping treatment on the gate insulating layer beforeforming the first oxide semiconductor film.
 8. The method formanufacturing a semiconductor device according to claim 6 wherein athickness of the second oxide semiconductor film is larger than athickness of the first oxide semiconductor film.
 9. The method formanufacturing a semiconductor device according to claim 6 furthercomprising a step of forming an oxide film over and in contact with thesource electrode layer, the drain electrode layer, and the second oxidesemiconductor film.
 10. The method for manufacturing a semiconductordevice according to claim 9 wherein the oxide film includes one or moreelements selected from constituent metal element of the second oxidesemiconductor film.
 11. A method for manufacturing a semiconductordevice, comprising the steps of: forming an insulating layer over asubstrate; forming a stacked oxide semiconductor film over theinsulating layer, the stacked oxide semiconductor film comprising afirst oxide semiconductor film and a second oxide semiconductor filmover the first oxide semiconductor film; performing oxygen dopingtreatment on the stacked oxide semiconductor film; forming a sourceelectrode layer and a drain electrode layer over the stacked oxidesemiconductor film; forming an oxide film over and in contact with thesource electrode layer, the drain electrode layer, and the second oxidesemiconductor film; and forming an electrode over the oxide film,wherein each of the first oxide semiconductor film and the second oxidesemiconductor film contains indium, wherein the first oxidesemiconductor film includes crystals which are c-axis alignedperpendicularly to a surface of the first oxide semiconductor film, andwherein the second oxide semiconductor film includes crystals which arec-axis aligned perpendicularly to a surface of the second oxidesemiconductor film.
 12. The method for manufacturing a semiconductordevice according to claim 11, wherein a thickness of the second oxidesemiconductor film is larger than a thickness of the first oxidesemiconductor film.
 13. The method for manufacturing a semiconductordevice according to claim 11, wherein the oxide film includes one ormore elements selected from constituent metal element of the secondoxide semiconductor film.
 14. The method for manufacturing asemiconductor device according to claim 11, further comprising a step offorming a gate electrode before forming the insulating layer.
 15. Themethod for manufacturing a semiconductor device according to claim 1,wherein each of the first oxide semiconductor film and the second oxidesemiconductor film is formed by sputtering while a temperature of thesubstrate is set to higher than or equal to 200° C. and lower than orequal to 500° C.
 16. The method for manufacturing a semiconductor deviceaccording to claim 6, wherein each of the first oxide semiconductor filmand the second oxide semiconductor film is formed by sputtering while atemperature of the substrate is set to higher than or equal to 200° C.and lower than or equal to 500° C.
 17. The method for manufacturing asemiconductor device according to claim 11, wherein each of the firstoxide semiconductor film and the second oxide semiconductor film isformed by sputtering while a temperature of the substrate is set tohigher than or equal to 200° C. and lower than or equal to 500° C.